Distribution of the temporal reference on an avionics platform of an aircraft conventionally may include several clock sources. Generally, there exist three clock sources adapted to generate and determine the temporal reference of the aircraft. The first source is a precise primary source arising from a GPS (Global Positioning System) satellite geolocation system integrated in general into a reception device called a Multi Mode Receiver MMR. The signal arising from the primary source is used as an input for the other two sources which are secondary sources. The secondary sources comprise Air Data Inertial Reference Units ADIRUs and optionally an LRU (Line Replaceable Unit) device of which the dedicated function is to distribute a clock: “LRU-Clock”.
The distribution of the temporal reference is carried out through an avionics network of deterministic switched Ethernet type, in accordance with the ARINC 664 standard, part 7. Such a network can for example correspond to an AFDX® communication network. The communications in the network are carried out on virtual links and each virtual link has a reserved end-to-end path through the network. Distribution of the temporal reference passes via a secure gateway named SCI (Secured Communication Interface) hosting a function dedicated to distribution.
However, for operational reasons certain devices receive a temporal reference through a specific bus and/or gateway. The choice of distribution depends on the validity and availability of the source. Furthermore, other devices can use yet other sources for maintenance needs through the SCI gateway. All these various sources and gateways increase the complexity of the architecture of the avionics platform and of the distribution function.
Moreover, due to the fact that the devices involved in this architecture are asynchronous and taking account of the latency times induced by the various gateways, the lag in forwarding the temporal reference is non-negligible and the inferred precision is limited. For example, the path linking the primary source (GPS), the secondary source (ADIRU-LRU-Clock), the AFDX network and the SCI gateway induces a latency of the order of a second. In case of fault of the LRU-Clock clock, the latency will be about double.
Furthermore, the multiplicity of sources and the complexity of the architecture decrease the optimization of the initialization time necessary for distribution of the temporal reference. Moreover, in case of partial or temporary loss of the platform or of the devices, the temporal reference may no longer be available.